Trummer began his motorsport career in karting in 2003, finishing second in the Swiss Junior Championship and placing 21st in the Andrea Margutti Trophy ICA Junior class. In 2006 he moved into single-seaters with Formula Lista Junior, finishing eighteenth overall. He joined BMS Böhlen Motorsport for the 2007 Formule Renault 2.0 Suisse championship, finishing seventh. Switching to Jenzer Motorsport for 2008, he improved substantially to finish as runner-up in the series.
Trummer moved to the International Formula Master series for 2009 with Iris Project, finishing eleventh in the standings. He then stepped up to the inaugural GP3 Series in 2010 with Jenzer Motorsport, recording 25th place with four points; he missed the Hungarian round after a collision at Hockenheim. For 2011 he moved to MW Arden, improving to 18th with nine points and recording three point-scoring finishes in the final rounds of the year.
Following a non-championship appearance at the GP2 Series season finale at Yas Marina in 2011, Trummer contested the GP2 Series full-time from 2012. He drove for Arden International in 2012, partnering Luiz Razia and finishing 23rd in the standings. He moved to Rapax for 2013, finishing 21st. His best GP2 campaign was 2014, again with Rapax, where he finished 17th overall and recorded his sole GP2 podium — second place in the sprint race at the Bahrain round. In 2015 he drove for Hilmer Motorsport, finishing 31st.
Alongside his GP2 campaigns, Trummer entered prototype endurance racing with the ByKolles/CLM team, competing under the Lotus branding in the LMP1 class of the FIA World Endurance Championship. He contested events in the 2014, 2015, and 2016 WEC seasons, including appearances at the 24 Hours of Le Mans in 2015, 2016, and 2017, each ending without a classified finish. The ByKolles entry was a privateer LMP1 programme and consistently faced performance and reliability challenges against the factory hybrid machinery.
From 2018 Trummer concentrated on North American prototype racing with JDC-Miller MotorSports in the IMSA WeatherTech SportsCar Championship. He finished 11th in the DPi class standings in 2018 and 8th in 2019.
His strongest IMSA season came in 2020, racing for PR1 Mathiasen Motorsports in the LMP2 class. Trummer won three races, took six pole positions, and finished as runner-up in the LMP2 class championship. He also competed at the 2021 24 Hours of Le Mans with the team in the WEC LMP2 class, retiring from that race as well.
In 2021 Trummer raced in the Asian Le Mans Series with Phoenix Racing, finishing fourth in the LMP2 class standings.
Trummer's career illustrates the typical trajectory of a well-funded Swiss single-seater prospect who finds a more durable niche in endurance prototype racing. His 2020 IMSA LMP2 title campaign — three wins and six poles — represents his most competitive season in terms of results. He remains active in endurance and long-distance series including the Nürburgring Langstrecken Serie.